1. Field of the Invention
This invention relates to an automatic transmission, particularly to an automatic transmission for an automobile which is used together with a torque converter, and more particularly to an automatic transmission mechanism which achieves four forward speeds by the use of two planetary gear units.
2. Description of the Related Art
Generally, an automatic transmission is formed of the so-called Simpson type including two single planetary gear units, a sun gear of both planetary units being commonly used and a carrier of the first planetary gear unit being integrally connected to a ring gear of the second planetary gear unit. In this type of conventional transmission, the four forward speeds are achieved as follows. That is, the driving force is selectively inputted by the ring gear of the first planetary gear unit and/or sun gear through a clutch, the predetermined rotational members of the planetary gear units are retained by a brake or a one-way clutch, and the driving force is outputted from the ring gear of the second planetary gear unit integrally connected to the carrier of the first planetary gear unit.
A conventional four speed automatic transmission employs the aforementioned three speed automatic transmission including two planetary gear units as a base, and additionally employs an overdrive (O/D) mechanism attached thereto and or underdrive (U/D) mechanism comprising a planetary gear unit in order to obtain a gear change stage for obtaining the four forward speeds.
In recent years, in view of the ever growing demand for F F (front engine front drive), high horse power, etc. in vehicles for example, the automatic transmission is required to be made much smaller. In addition, the transmitting torque capacity is required to be made larger. However, a four speed automatic transmission mentioned above with three planetary gear units including an additionally attached O/D mechanism etc. can't meet the demands mentioned above.
Under the circumstances, there is proposed an automatic transmission mechanism in which two clutches are additionally attached to the conventional so-called Simpson type planetary gear unit for obtaining four speeds in Japanese Laid-open Patent Application No. 59(1984)-183147. That is, the mechanism of the automatic transmission has a third clutch that is disposed between the sun gear of the first planetary gear unit and the sun gear of the second planetary gear unit integrally connected with respect to each other in the conventional Simpson type, at the same time, the sun gear of the first gear unit communicates with the carrier of the second gear unit through a fourth clutch, the third clutch is disengaged to separate the first gear unit from the second gear unit, the fourth clutch is connected in order to transfer an input to the carrier of the second gear unit, and at the same time, the sun gear of the second gear unit is fixed, in order to output an overdrive from the ring gear to obtain a fourth speed in addition to the three speeds.
However, since both the planetary gear units are separated by a clutch in the improved Simpson type automatic transmission, it is difficult to dispose both the planetary gear units in the integrated state. Moreover, it is difficult to dispose many clutches together at one place. Accordingly, in order to modify the conventional Simpson type three speed automatic transmission mechanism to a four speed automatic mechanism by additionally attaching third and fourth clutches thereto, an extremely large modification is required, and the three speed automatic transmission mechanism and four speed automatic transmission mechanism have to be manufactured separately. Due to the foregoing, common use of parts and a assembly line are difficult to attain, and a small production of many kinds of parts often invite a significant increase in cost, thus inhibiting the requirements of recent time tendency of various kinds of vehicles and wide variations thereof.
On the other hand, attention is paid to the so-called Ravigneaux type automatic transmission mechanism, wherein two units of carriers are integrally formed and which is expected to be made much smaller.
The Ravigneaux type automatic transmission mechanism 1, as shown in FIG. 11, comprises a single planetary gear unit 2' and a dual planetary gear unit 3'. An input member I is connected to a sun gear S.sub.2 of the dual unit 3' through a clutch C.sub.1, and also to a sun gear S.sub.1 of the single unit 2' through a clutch C.sub.2 or a one-way clutch F.sub.o. The input member I is further connected to a carrier CR.sub.2 supporting dual pinions P.sub.2, P'.sub.2 and to a first carrier CR.sub.1 which is integrally connected to the second carrier CR.sub.2, through a clutch C.sub.o. An output member O is connected to ring gears R.sub.1 and R.sub.2 of both the units 2' and 3' which are connected with each other and are integrally rotated.
The Ravigneaux type automatic transmission, as shown by an operation table of FIG. 12, offers one speed through four speed (1st through 4th) and reverse (Rev) according to actuation of the respective clutches C.sub.1, C.sub.2 and C.sub.o, the respective brakes B.sub.1, B.sub.2 and B.sub.3 and the respective one-way clutches F.sub.1, F.sub.2 and F.sub.o. In the figure, .circle. denotes coupling of clutches and actuation of brakes and one-way clutches, .DELTA. denotes the actuation during only coast time, and .circle..cndot. denotes actuation only when gears are shifted and thereafter synchronous rotation is effected.
However, in the Ravigneaux type automatic transmission mechanism, since engine torque is input to the sun gear S.sub.2 (S.sub.1), the tangential force acting on the sun gear with a smaller diameter is high and particularly this force significantly appears first speed at which the transmitting torque is large. On the other hand, there are problems of the durability of the gear face of the sun gear S.sub.2 not being satisfactory. Also, the durability of a brake B.sub.3 and one-way clutch F.sub.2 which often receive the reaction force of the carriers CR.sub.1 and CR.sub.2 is problematical. Because of the foregoing, allowable input torque is limited, thus not meeting the recent time for high horse power. Any attempt to make the allowable input torque larger results in a need for thickening the gear width of the sun gear. Furthermore, tolerance of retaining means such as brakes, etc. is required to be made large which again results in a large size of an automatic transmission, and problems arising in respect to loading them on vehicles.